<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>删除</button>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
    </ul>
    <script>
        var btn = document.querySelector('button')
        var ul = document.querySelector('ul')
        //删除元素
        btn.onclick = function () {
            if (ul.children.length == 0) {
                this.disabled = true;  //如果ul里的长度为0 按钮变不可选
            }
            else {
                ul.removeChild(ul.children[0])
            }
        }


    </script>
</body>

</html>